For a long time, the
classification of brain tumors has been based on
histogenesis. The success of the classification
proposed by Bailey and Cushing in 1926 has been
associated with the hypothesis that the supposed
cell of origin and the degree of maturation of
tumor cells are determinant factors for
prognosis. Histopathologic criteria,
immunohistochemistry and, more recently,
molecular genetics have improved
characterization of the supposed cell of origin
and have further refined the prognostic
criteria. The debate about the origin of brain
tumors is again of great interest after the
introduction of neural stem cells and their
implications for new therapeutic strategies.
The natural history of brain tumors can be
divided into four main stages: initiation,
progression, transformation, and diffusion. The
initiation of glial tumor is not yet understood,
and is the focus of intense research. When a
macroscopic lesion is first identified in
humans, it is already organized as a tumor.
Experimental studies in rats have shown that
induced tumors arise from primitive
neuroepithelial (neural stem) cells of the
ventricular zone (VZ), from its derivative
subventricular zone (SVZ), or from a renewal
pool of cells in the remnants of SVZ or
subependymal layer, hippocampus, cerebellum, and
first cortical layer. Neural stem cells are
characterized by their self-renewal capability
and multipotency.
Both neurons and glial cells derive from neural
stem cells of the VZ and SVZ, and they
differentiate along their respective pathways
under extrinsic and intrinsic stimuli. Growth
factor signaling also controls their passage
from one stage to the next. Availability of
markers for glia or neurons affirms that
neurogenesis and gliogenesis continue in the
adult mammalian brain; demonstration of the
existence of neural stem cells also in the adult
has changed our notion of oncogenesis
dramatically. Glial tumors may arise from stem
cells either in the VZ or SVZ. New hypotheses on
glioma origin postulate that astrocytes and
radial glia (astroglial lineage) might act as
multipotent stem cells both in embryo and in
adulthood. The vulnerability of stem cells to
undergo neoplastic transformation depends on the
interaction of several factors, including the
number of replicating cells, the duration a cell
population remaining in cycle, and the state of
cellular differentiation.
Another important concept is that the more
genetic alterations are needed for a tumor to
develop the more advanced the progenitor cells’
stage of differentiation. Then the histology of
a tumor may be more a reflection of the
environment and time of initiation than of the
cell of origin, and the former factors would
determine whether a tumor ultimately becomes an
astrocytoma or an oligodendroglioma. Early
genetic events differ between astrocytic and
oligodendroglial neoplasms, but all tumors have
an initially invasive phenotype, which
complicates therapy. Tumor cells are
heterogeneous and variably express
differentiated antigens typical of the cell of
origin, but only a minority of them are
self-renewing, multipotent, clonogenic, and
continuously replenishing mature cells. The
discovery that only a proportion of tumor cells
are clonogenic when xenografted has introduced
another important new concept: the existence of
cancerous stem cells.
The growth and transformation of tumors can be
summarized as follows. Tumors are supposed to
initiate as monoclonal expansion and, due to
genetic instability, they will develop genetic
heterogeneity that is associated with increasing
mutation rate and proliferation capacity. Once
the genome becomes unstable, as the cell
divides, genetic material that codes for growth
promotion (i.e. oncogenes of which protein
products serve to accelerate cell growth) will
accumulate, whereas genetic material that codes
for growth control (i.e. suppressor genes of
which protein products serve as brakes on cell
growth) will be lost. These events result in
phenotypic heterogeneity. New clones will arise
and compete with older clones. They may adapt
better to the tissue microenvironment and show
greater proliferation potential after having
lost differentiating capacity. Anaplasia is a
feature of the neoplastic tissue that has lost
such capacity typical of a given cytogenetic
stage, and regresses to a less differentiated
stage. Various molecular genetic alterations
have been linked to the following pathologic
events occurring with tumor progression:
proliferation and apoptosis in diffuse
astrocytoma; deregulation of cell cycle in
anaplastic astrocytoma; necrosis, angiogenesis,
and clonal selection in glioblastoma (GBM).
Cell migration and invasion, angiogenesis,
necrosis, and apoptosis are neoplastic events
that have been consistently associated with poor
prognosis in gliomas. New therapeutic strategies
can be explored when the molecular pathways
regulating these events and their phenotypic
imaging characteristics become better
understood.
Several molecular biology techniques are used to
identify genetic aberrations in tumors. At
present, the most commonly applied technique is
genotype analysis with fluorescent
microsatellite markers that evaluate loss of
heterozygosity (LOH). Preliminary results
suggest that in the near future, molecular
genetic tumor analysis together with clinical,
quantitative imaging and histopathologic
phenotypes will allow a more accurate prediction
of survival and will be of great importance in
selecting and developing the appropriate
therapy. At present, the use of quantitative
imaging and molecular genetics for the
classification of the tumor type, subtype, or
grade remains a challenge because large
validation studies are still lacking. It is also
unknown if the correlation found between
molecular genetics data and survival can be
prospectively applied to each individual case at
the time of diagnosis.
Growth depends on the balance between cell
proliferation and cell loss, and is regulated by
cell cycle time, growth fraction, tumor doubling
time, necrosis, and apoptosis. Necrosis is a
sudden event in which many cells are killed at
the same time because of hypoxia, energy
depletion, and inflammatory response. Apoptosis
denotes a programmed cell death comprised of
three separate complex regulatory pathways, and
is considered the major cause of cell loss in
gliomas and other tumors. However, apoptosis is
more often associated with a high proliferation
rate. The apoptotic index increases in the
spectrum from low-grade diffuse (infiltrative)
astrocytoma to GBM; in oligodendroglioma, the
index is higher than in astrocytoma, and it also
increases with anaplasia. Moreover, a high
apoptotic index is found in PNETs, lymphomas,
and metastases. Whether the finding of increased
number of cells undergoing apoptosis might
indicate tumor regression and better prognosis
is still controversial. Failure of apoptosis may
be responsible for tumor development and may be
linked to breakage of the pathway regulated by
tumor suppressor p53. In contrast, induction of
apoptosis in glioma cells could be instrumental
to therapies.
Diffusion and infiltration into the adjacent
brain tissue are two other important properties
of gliomas and make their treatment much more
difficult. Motility of glioma cells and invasion
are facilitated by extracellular matrix and
adhesion molecules. Cell motility is associated
with poor prognosis. Velocity of solid tumor
expansion is linear with time and varies from 4
mm/
year in low-grade glioma (LGG) to 3mm/month in
high-grade glioma (HGG). There is a cell density
gradient decreasing from the center of the mass
towards its periphery at the boundary with
normal tissue. How far neoplastic cells can be
found from the
macroscopic edge of the tumor is another very
important issue. The 2 cm distance detected by
computerized tomography has been considered the
safety margin according to the classic report by
Burger in 1988.
It is about time that new studies with more
updated imaging modalities define with greater
accuracy the extent of a tumor. Cell motility
and invasion capability have great implications
for planning of surgery and post-surgical
adjunct therapy.
Cell invasion is not necessarily a consequence
of cell proliferation. Examples of infiltrative
but not highly proliferative gliomas are common
and have been described. In solid tumors, a
cellular density gradient is much more
frequently found between the center and the
cortex than toward the white matter. Similar
gradient properties can be found in the
frequency of mitoses and nuclei stained for
proliferation markers, MIB1. When the tumor
border is clear-cut the gradient is also steep.
In infiltrated cortex, the MIB1 labeling index
may be very low, because there is a dissociation
between migratory and proliferation capacities.
Recognition of asymmetry in tumor infiltration
and proliferation also has implications for
therapy planning.

Histopathologic classification
of brain tumors

The classification of tumors of the nervous
system released by the World Health Organization (WHO) in 2007
is the most comprehensive to date and has received a large
consensus among neuropathologists. It divides brain tumors into
seven categories: tumors arising from neuroepithelial tissue,
from peripheral nerves, from the meninges, lymphomas and
hematopoietic neoplasms, germ cell tumors, tumors of the sellar
region, and metastatic tumors.
Among tumors of neuroepithelial tissue, the gliomas are by far
the most common and best studied.
Gliomas include tumors arising from neural stem cells in the VZ
or SVZ or from neoplastic transformation of precursor or mature
glial cells. The group of neuroepithelial tumors also includes
ependymomas, neuronal and mixed neuronal–glial tumors (such as
ganglioglioma), and embryonal tumors (such as medulloblastoma).
Gliomas are classified by their histologic features, according
to the presumptive cell of origin, differentiation, and grade of
malignancy. At the current state of knowledge, cytogenesis is
more a theoretical concept than a definitive basis for tumor
classification.

Astrocytomas are believed to originate from astrocytes, which are
stellate branched cells permeating the interstitium and
interacting with blood vessels and neurons. Astrocytes have
indeed a very important stromal role: the extracellular matrix
reacts through cell adhesion molecules and other factors with
the intracellular space, modulating cell migration,
differentiation, proliferation, and apoptosis. In view of the
abundance of astrocytes and their multiple functions,
astrocytomas make up a heterogeneous group of tumor subtypes,
with different biological behavior. According to the WHO
classification, glial tumors are graded on the basis of the most
malignant area identified on histopathologic specimens.
Fibrillary astrocytoma (WHO grade II) is characterized by
increased cellularity with a monomorphic population of cells
infiltrating the neuropil. Anaplastic astrocytoma (WHO grade
III) is characterized by nuclear polymorphism and mitoses. The
occurrence of angiogenesis and necrosis are features of GBM
(grade IV). GBM can arise de novo (primary) or transform from a
pre-existing LGG (secondary).
Anatomic location and patient’s age at initial presentation are
also very important factors for diagnosis and prognosis.
According to a recent study by Duffau and Capelle, LGGs more
frequently than GBMs are found in the cortex of secondary
functional areas, especially within the supplementary motor area
(SMA) and insula.[15] This preferential location may be
associated with a high risk of adverse postoperative sequelae,
and it may be one reason that resection of LGGs remains
controversial.

Oligodendrogliomas
are moderately cellular and composed of monotonous, round, and
homogeneous nuclei with a clear cytoplasm. They have a dense
network of branching capillaries. It is not uncommon for
oligodendrogliomas to bleed, and they may present as an
intracranial hemorrhage. Additional histologic features include
calcifications and mucoid/cystic degeneration. Estimates of
incidence of oligodendroglioma vary enormously in different
series since diagnosis
depends on use of permissive or restrictive criteria. If
restricted histologic criteria are used, diagnostic signs are
the honeycomb appearance of the cells and the high density and
chicken-wire distribution of small vessels.
When more permissive criteria are used, the incidence of
oligodendrogliomas obviously increases while that of diffuse
astrocytomas decreases. Burger has masterly illustrated this
diagnostic conflict. Today more than ever, the correct diagnosis
of oligodendroglioma is important because effective chemotherapy
has become available. Allelic LOH in the 1p and 19q chromosomes
have been associated with longer survival and a favorable
response to chemotherapy with procarbazine, lomustine, and
vincristine (PVC). In contrast, LOH on 17p and TP53 mutations
characteristic of astrocytic tumors are rare in
oligodendrogliomas and practically mutually exclusive with LOH
1p and 19q. The differential diagnosis also has important
prognostic implications, since the number of mitoses and nuclei
positive for proliferation markers found in the two tumors may
receive a different weight by the neuropathologists. In diffuse
astrocytomas, mitoses are absent or very low in number and a
high mitotic index indicates anaplasia. In grade II
oligodendrogliomas, the number of mitoses allowed, on the other
hand, is definitely higher. Therefore the same mitotic index may
suggest anaplasia or not depending on the diagnosis of
astrocytoma or oligodendroglioma. The apoptotic index is also
much higher in oligoastrocytoma than in astrocytomas. As we will
see later, this asynchronous biologic behavior between the two
tumor types is a recurrent confounding factor in neuroimaging as
well: the choline (Cho) signal measured with 1H-MRS and the
cerebral blood volume (rCBV) measured with perfusion MR are
generally higher in grade II oligodendrogliomas than in
astrocytomas.

Mixed
oligoastrocytoma (MOA) were first recognized as an
entity by Cooper in 1935. The diagnosis of MOA requires
recognition of two different glial components, both of which
must be unequivocally neoplastic. They may be divided into
biphasic (compact) and intermingled (diffuse) variants. In the
former, distinct areas of both cell types are juxtaposed, while
in the latter variant the two components are intimately admixed.
Estimates of their incidence vary with the diagnostic criteria
used and must be interpreted with caution.
Their incidence may vary from 1.8% in North American, 9.2% in
Norwegian, and 10–19% in German series. On conventional MRI, MOA
demonstrate no special features that would allow a reliable
distinction from oligodendrogliomas. About 30–50% of MOA are
characterized by LOH on 1p and 19q. About 30% carry mutations of
the TP53 gene and/or LOH on 17p that are frequently found in
astrocytomas. The presence of LOH 10q has also been associated
with shorter overall survival in MOA. Apparently these genetic
alterations are consistent throughout every individual MOA and
suggest that they are monoclonal neoplasms originating from a
single precursor cell rather than tumors that have developed
concurrently.

GBM is
the most malignant astrocytic tumor, composed of poorly
differentiated neoplastic cells. It is the most frequent brain
tumor, accounting for 12–15% of all intracranial tumors. GBM may
manifest at any age, but about 80% of patients are between 45
and 80 years old. It may develop from diffuse astrocytomas,
anaplastic astrocytomas, but more frequently they occur de novo
after a short clinical history. Primary GBM accounts for the
vast majority of cases in older people, while secondary GBM
typically develops in younger patients (less than 45 years). The
time to progression varies considerably, ranging from less than
1 year to more than 10 years, with a mean interval of 4–5 years.
There is increasing evidence that these two subtypes represent
distinct disease entities, which evolve through different
genetic pathways, and are likely to respond differently to
therapy. GBM is a very heterogeneous disease and it is often
multifocal. Multifocal GBM is suggestive of a more invasive and
migratory tumor phenotype, a feature more common to stem cell
derived cancer. According to a recent report GBM originating
from the SVZ and extending into the cortex has a higher rate of
developing multifocal disease, while GBM growing in the cortex
has a higher rate of local recurrence. Tumor location may
suggest the presumed origin of the tumor from SVZ neural stem
cells in the former phenotype and from transformation of
mature glial cells in the latter. Stem cell-derived GBM may
require treatment that attends to both the primary lesion and
the SVZ.

WHO grading and patient
survival

Histological grading is not only a practical
way of predicting the biological behavior of a tumor in an
individual patient, but also an effort to define, in conjunction
with other parameters, a homogeneous group of patients.
Significant indicators of anaplasia in gliomas include nuclear
atypia, mitotic activity, cellularity, vascular proliferation,
and necrosis. These histopathological features are condensed in
a three-tiered scheme. A simple and reproducible grading scheme
is of paramount importance to planning therapy as well as for
the interpretation of response to multiple therapeutic regimens.
The validity and reproducibility of any grading system depends
on the homogeneity of the lesions within each class. Grading is
only one component of a combination of criteria used to predict
a response to therapy and outcome. Other criteria include the
patient’s age, neurological performance status, tumor location,
extent of surgical resection, proliferation indices, genetic
alterations, and one radiological feature: contrast enhancement.
Whether 1H-MRSI and other imaging parameters will be included in
this list will depend on their unique contribution to
characterize neoplasms in a homogeneous group. The two biggest
assets of in vivo MR imaging are the possibility to follow
non-invasively the biological behavior of individual tumors and
guide therapy.
For each tumor type, the combination of the above criteria
contribute to an overall approximation of prognosis. The median
survival for grade II diffuse astrocytoma is around 5 years;
however, the range of survival is broad and unpredictable.
Despite the initial low proliferative index, most of these
patients die from progression to GBM. Patients with grade III
anaplastic astrocytoma survive for 2–3 years; the majority of
patients with GBM, in particular the elderly, have a median
survival of less than 1 year. The outcome of 676 GBM patients
over a 7-year period at a single institution has been reported
recently: survival probabilities were 57% at 1 year, 16% at 2
years, and
7% at 3 years.
Studies of patients with grade II oligodendrogliomas reported a
median survival of about 10 years. A recent series of 106
patients yielded a median survival of 16 years, probably due to
earlier diagnosis following the advent of MRI. Malignant
progression is not uncommon, although it is considered less
frequent compared to diffuse astrocytomas. A median survival
time of 6.3 years and 5- and 10-year survival rates of 58% and
32%, respectively, have been reported in a study of 60 patients
with grade II MOA.